We use potential outcomes to define causal effects [1]}, [2]}.
For each beneficiary \(i\)  in practice \(j\) , let \(Y_{ijt}(1)\)  denote their potential outcome at time \(t\)  when their practice was treated between years 2 and 3. Similarly, let \(Y_{ijt}(0)\)  denote their potential outcome at time \(t\)  when their practice was untreated between years 2 and 3The potential outcomes notation makes some implicit assumptions, notably (i) treatment status of practice \(j\)  cannot affect the potential outcome of beneficiaries in practice \(j^{\prime } \ne j\) , (ii) there are no different versions of treatment (or non-treatment), (iii) treatment status does not vary amongst beneficiaries in the same practice, and (iv) treatment status does not change once assigned in between years 2 and 3.
Conditions (i) and (ii) are often known as the Stable Unit Treatment Value Assumption [3]}, [4]}.
